And who was going to remember the colonists, considering that a week before, know one in the galaxy knew about them?
Well, the whole galaxy, supposedly. Why would Picard keep it a secret? Out of spite?
Huh? Sounds like you;re saying that if you are attacked in war, the best choice is to surrender, because the attackers are probably going to win!
Well, generally this is true for modern wars of conquest (which aren't all that common, but certain periods of history have lots of those as opposed to the other sorts of war). If the attacker didn't have both the means and the will to crush you, he wouldn't have started the war. You're most probably much better off if you do not try to defend yourself, or at least give up after going through some motions.
However, wars rarely exist in isolation. Even if defense of Poland against Germany is hopeless from the viewpoint of Poland itself, it poses advantages for the other enemies of Germany, and therefore for Poland, too...
But if a nation today feels strong enough to take on another nation, we're generally talking about entities of such magnitude that all but the most powerful alliances will hesitate to step in. History appears to fluctuate in this sense: there are periods where the balance of power is such that any defense is symbolic only and will not directly benefit the defenders, and periods where the sovereignty of a nation will be protected by interested outside parties for reasons that might also be considered symbolic.